The present invention is related to a shim and step pin assembly and more particularly to a step pin assembly having a self aligning shim for providing a compression pre-load to a forked mounting flange.
Construction equipment has many component parts that move relative to another machine part to perform the work of the machine. The moving component parts are often rotated about a pivot joint such as a pivot pin. Linear actuators, such as hydraulic cylinders often cause rotation of the parts. For example, a backhoe or excavator has a boom arm that pivots about a boom base on the vehicle body. A dipper arm then pivots relative to the boom arm at a pivot joint on the distal end of the boom arm.
A known pivot joint for connecting the boom arm and the dipper arm is constructed with a pivot pin through a boom arm having two, generally parallel, spaced-apart mounting flanges. A pair of aligned bores are provided in the mounting flanges. The dipper arm is constructed with a mounting portion having a lateral dimension sized to fit between the mounting flanges. The mounting portion also has a lateral bore. When the mounting portion is positioned between the spaced-apart flanges, the bore in the mounting portion is aligned with the bores in the flanges.
The hardened pivot pin is then inserted through the aligned bores to allow the dipper arm to pivot relative to the boom arm. The pivot pin is axially retained within the mounting flanges by a combination of locking pins, snap rings, threaded fasteners or an enlarged head on one end of the pin.
A known boom arm is preferably constructed as an integral cast member from a cast metal such as ductile iron. A cast boom arm is strong, economic and requires less assembly than a boom arm assembled from metal plates, for example. The cast boom arm has a pair of vertically orientated wall portions with integral cross-supports. The distal end of the wall portions are extended and left open to form cantilevered mounting flanges for pivotal connection of a dipper arm member.
A boom arm and especially the mounting flanges of the boom arm are often subjected to twisting or torsion stress during use. For example, twisting stress occurs on the mounting flange portion of the boom arm when the bucket on the extended dipper arm encounters a sideways force, such as when the bucket is swung into the side of a trench. It is therefore necessary to reinforce the mounting flange portion of a boom arm to resist this twisting or torsion stress. Adding additional structure, which adds weight and increases manufacturing costs, is not the preferred mechanism for reinforcing the mounting flange portion.
The mounting flange portion of a cast boom arm structure can be preloaded with a favorable compression stress to counteract the undesirable twisting stress. To preload a cast boom arm against twisting stress, the lateral gap between the mounting flanges is designed and constructed to initially be oversized. An oversized lateral gap between the mounting flanges has a larger opening than is required to fit the lateral width of the mounting portion of the dipper arm.
The two cantilevered mounting flanges of the cast boom arm are then pulled together by the pivot pin and fastener, such as with a threaded pivot pin and nut, to reduce the final lateral span in the oversized gap. Reducing the final span also provides a snug non-binding fit for the dipper arm between the mounting flanges of the boom arm. When the nut is tightened on the pivot pin and closes the initial oversized gap between the mounting flanges, the tightening creates a compression pre-load on the boom arm. The compression pre-load on the flanges counteracts the undesirable twisting stress that acts on the mounting flanges of the boom arm during operation of the backhoe.
It is difficult to control casting dimensions for large castings such as a boom arm. The dimensions of the lateral gap between the mounting flanges are difficult to manufacture within reasonable tolerances. To avoid extra machining, the manufacturing tolerances for the mounting flanges on the known cast boom arm are designed so that the tolerance stack-up due to casting variations always produces at least the desired oversized gap. Shims are used to take up the oversized portion of the lateral gap created by the tolerance stack-up.
A step pin, rather than a straight pivot pin, is used with the known cast boom arm to achieve the desired compression preload and the final lateral gap dimension between the mounting flanges. The step pin is manufactured to have a perpendicular shoulder at a predetermined axial position along the pin. The shoulder defines a reduced diameter portion having threads for engagement with a threaded nut. The axial position of the shoulder is designed so that the nut can be threaded to a final tight position on the step pin. The tight position of the nut against the shoulder squeezes the mounting flanges together to produce the desire compression pre-load. The tight position of the nut also results in the proper lateral gap between the flanges to snugly position the dipper arm in the boom arm.
To produce the desired compression pre-load, one end of the step pin is axially fixed relative to a first flange. The shank portion of the step pin extends through the second flange. An annular shim having an appropriate thickness and with a large inner diameter that fits outside the diameter of the shank portion is positioned outside the second flange. A load-transmitting washer is positioned on the reduced diameter portion of the pin adjacent to the shoulder and axially outward from the shim. As a nut is advanced on the threads of the reduced diameter portion, the washer is forced by the nut into a final contact position with the shoulder to squeeze the mounting flanges to the predetermined final lateral dimension. The shim is positioned between the washer and the mounting flange to take up the lateral space created by tolerance variations.
During assembly, however, a flat shim with a large inner diameter can shift off center and onto the shank portion of the step pin and move into the space on the reduced diameter portion of the step pin between the pin shoulder and the advancing washer. In that situation, the washer squeezes the intervening shim against the shoulder. The final position of the washer is not directly against the shoulder, which results in a larger lateral gap between the mounting flanges than the designed final lateral gap. The desired compression pre-load is not applied to the mounting flanges. As a result, subsequent twisting forces that act on the boom arm could cause a fracture at the mounting flanges.
Thus one object of the present invention is to provide a shim structure that provides for self aligning or centering of the shim outside the shank portion of the step pin during assembly so that the desired compression pre-load is applied to the mounting flanges of the boom arm.
Another object of the present invention is to provide a stackable shim that can provide the proper compression pre-load to the mounting flanges of the boom arm so as to resist undesirable twisting stress on the boom arm.
In a preferred embodiment, the present invention is an annular shim for use in combination with a pin assembly. The pin assembly includes a step pin, a washer and a threaded nut. The step pin has a shank portion with first diameter and a reduced diameter portion defined by a shoulder. The reduced diameter portion has threads for engagement with the threaded nut. The washer fits on the reduced diameter portion between the shoulder and the threaded nut. The annular shim includes an annular shim body having an inner diameter with a flat portion adjacent the inner diameter and an outer diameter with a cupped portion adjacent the outer diameter. The inner diameter fits outside the first diameter of the shank portion. The cupped portion receives the washer so as to center the inner diameter outside the shank portion and resist radial movement of the annular shim onto the shank portion as the threaded nut moves the washer into abutting contact with the shoulder.
In another embodiment, the present invention is a step pin assembly for supporting pivotal rotation of a first member relative to a second member. The first member has a pair of laterally spaced, generally parallel flanges having aligned first and second bores. The second member has a lateral bore, the flanges defining an oversized lateral gap to receive the second member when the bore of the second member is axially aligned with the aligned bores of the flanges. The step pin assembly includes a step pin having a first end, a second end and an axially extending shank portion therebetween. The first end is axially retained relative to one flange. The shank portion has a shank diameter for insertion through the axially aligned bores. The second end has a reduced diameter portion defined by a shoulder on the shank portion and has threads. The reduced diameter portion extends axially through the other flange when the shank portion is inserted into the aligned bores. A washer has an outer edge with a diameter larger than the shank diameter and an inner orifice adapted to fit on the reduced diameter portion of the step pin and move axially toward the shoulder. At least one annular shim member is positioned between the washer and the other flange. The shim member has an inner diameter and an outer diameter. The inner diameter fits outside the shank diameter. The shim has a flat portion at the inner diameter and a cupped portion at the outer diameter adapted to receive the outer edge of the washer within the cupped portion so as to restrict radial movement of the shim. A fastener is positioned adjacent the washer and has threads for engagement with the threads of the reduced diameter portion so as to threadingly move the washer into abutting contact with the shoulder of the step pin and thereby force the shim into abutting contact with the other flange.